1. Field of the Invention
The present invention relates to power transmission pulleys, and more particularly, to timing belt pulleys with noise-suppressing features.
2. Description of Related Art
V-belt drive systems are known to be less efficient than timing belt (toothed) drive belt systems. This is chiefly due to belt slippage in V-belt systems. The teeth on the timing belt and pulley prevent such slipping in timing belt systems. In some applications, a 5-10% energy savings may be achieved by replacing a V-belt drive system with a timing belt drive system. This energy savings is very significant in high-power transmission systems.
When it is desirable to use a timing belt power transmission in applications where high speeds and high power transmission are required, relatively wide belts must be used. However, such wide belts create significant noise problems in the form of a loud whistle. This noise is believed to be caused by the rapid escape of air trapped between the pulley and the belt where the pulley teeth engage belt teeth and the reverse effect where the belt disengages from the pulley.
In many hospitals, high horsepower belt drive transmissions are used to drive fans which ventilate the air in the rooms. Ventilation of hospital room air is particularly important to reduce the number of airborne viruses and bacteria. Special requirements put forth by the Joint Commission on Accreditation of Health Care Organizations specify the minimum number of total air exchanges per hour for hospital rooms. For example, the air in trauma rooms, operating rooms and delivery rooms must be exchanged at least 15 times each hour requiring a great deal of power. Thus, hospitals typically employ the very highest powered belt transmissions available for driving their ventilation fans. A small increase in efficiency in these systems will result in a large economic return.
While hospital ventilation systems provide a particularly good application for higher-efficiency timing belts, in many cases, timing belt transmissions cannot be used due to their greater noise generation. The noise generated by a large timing belt transmission may be transmitted some distance through the air ducts to the patients' rooms. In one hospital, a newly installed timing belt transmission had to be replaced with a less-efficient V-belt transmission as a result of complaints of patients and hospital personnel.
Several attempts have been made to overcome the problem of noise in timing belts. In one proposal, the pulley was provided with a peripheral section made of sintered, porous metal. The trapped air between the belt and the pulley was permitted to escape through the pores in the metal. This proposal was expensive and difficult to manufacture.
Several proposals for curing the noise problem are disclosed in U.S. Pat. No. 4,545,778. Several pulley constructions are shown in this patent which purport to reduce noise by permitting air trapped between the pulley and the belt to escape via passages formed in the pulley face. A model similar to the pulley of FIG. 11 in the aforementioned patent was tested by the present inventor. This model provided some noise reduction at some noise frequencies as compared to a standard timing pulley, but failed to provide enough noise reduction for many hospital and other noise-sensitive installations. In particular, this model failed to satisfactorily decrease noise levels over a wide range of disturbing noise frequencies.